1. Field of the Invention
The present invention relates to a wafer mapping system that detects the presence or absence of wafers accommodated in a wafer accommodation container (a cassette).
2. Description of the Related Art
When a wafer such as a semiconductor wafer is transferred in a fabricating process of a semiconductor product, usually a plurality of wafers is accommodated in a wafer accommodation container (hereinafter referred to as a cassette) and transferred between the respective steps. In a wafer processor, a necessary processing is applied to cassette by cassette. In this case, preceding the processing with the wafer processor, it is necessary to grasp an accommodation state of wafers for each cassette. Accordingly, in a wafer processor that processes wafers in a cassette, a wafer mapping system is disposed to detect the presence or absence of wafers. As the wafer mapping system, there is one that is disclosed in, for instance, JP-A-3-297156.
In the wafer mapping system disclosed in the publication, as a cassette for accommodating a plurality of wafers, a cassette called an open cassette is used. In the cassette, at a front side thereof an access port is opened to allow inserting and taking out a wafer, at a back side thereof an access port smaller than the access port is disposed. Furthermore, inside of the cassette, slots are formed in a plurality of tiers so as to hold the wafers substantially level. In a cassette susceptor where a cassette is placed prior to the processing of wafers, wafer detection means for identifying a wafer accommodation state in advance of the processing of the wafer is disposed. The wafer detection means is constituted of a transmission type sensor formed of a light-illuminating element and a light-receiving element that are disposed oppositely with a cassette disposed on a cassette susceptor sandwiched therebetween. When the transmission type sensor is moved in a up and down direction from a slot in the uppermost tier to one in the lowermost tier in the cassette, the presence or absence of a wafer accommodated in each of the slots in the cassette can be grasped.
However, in recent years, there is a tendency in a larger wafer size, accompanying this tendency, a new specification is stipulated of a cassette for accommodating wafers. A cassette in conformity to the specification is called a FOUP (Front Opening Unified Pod). In the FOUP, only one access port is disposed to insert and take out a wafer and to the access port a lid is detachably attached.
In the FOUP, since there is only one access port, different from the disclosure in JP-A-3-297156, the presence or absence of a wafer accommodated in each of slots of a cassette cannot be detected with the transmission type sensor that sandwiches the cassette fore and aft. Accordingly, in order to detect the presence of a wafer, a motion of once opening a lid attached to the access port becomes anyway inevitable. Because of the necessity of the motion, a motion for detecting the wafer becomes complicated, and a wafer mapping system tends to become complicated. In addition, there are various restrictions such as a wafer being accurately detected with difficulty and so on.
As a device that can alleviate these restrictions, a device that is disclosed in, for instance, U.S. Pat. No. 6,188,323 is known. In the device, after a lid of the FOUP is opened, with a mechanism that removes the lid lowering and retracting, a sensor formed of a pair of an illuminator and a photo-receiver that are attached to the mechanism detects the presence or absence of a wafer sequentially from top down in the cassette. The wafer can be detected by intercepting a light beam from a transmitter to a receiver.
However, even in the device disclosed in U.S. Pat. No. 6,188,323, to the mechanism that removes the lid, the respective sensors of a light emitting side and light receiving side have to be attached. Accordingly, it is inevitable that a structure of the wafer mapping system becomes complicated to a certain degree.
Furthermore, since in the wafer mapping system, both an illuminator and a light receiver, while moving, detect a wafer, a detection motion tends to be unstable. Accordingly, a high accuracy wafer mapping system that can accurately detect a wafer with more stability is demanded.
Furthermore, in such a case of a wafer being improperly accommodated, the wafer mapping system can properly detect the wafer with difficulty. FIG. 25 is a diagram showing a wafer accommodation state in a cassette 100 from an access port 101 side of the cassette 100, part of the cassette 100 being shown together with wafers 102 that are held and accommodated in the respective slots thereof. Of wafers 102a, 102d, and 102e and voids 103a through 103c shown in the drawing, with the wafer mapping system, the presence or absence of a wafer can be detected. However, in the wafer mapping system, with an illuminator and a light receiver simultaneously moving, the detection is performed depending on whether a light beam is intercepted by a wafer or not. Accordingly, of the wafer 102b in a multi-slotted state and the wafer 102c in a cross-slotted state, the wafer accommodation state cannot be properly detected.